1. Field of the Invention
The present invention relates to a method and apparatus for measuring a carrier lifetime of IV group semiconductors.
2. Description of the Related Art
It is known that carrier lifetime plays a dominant role in determining IV group semiconductor (mainly silicon) device characteristics, such as current versus voltage and switching characteristics. Therefore, many methods have been proposed to evaluate carrier lifetime. The first method among the proposed ones is photo-conductivity, which is disclosed in, for example, G. K. Wertheim and W. M. Augstyniak, Review of Scientific Instruments, vol. 27 1956, p.106. The second is reverse recovery time of a diode current, which is disclosed in, for example, B. Lax and S. F. Neustadter, Journal of Applied Physics, vol 25, 1954, p.1148. The third is open circuit voltage decay of a diode, which is disclosed in, for example, S. R. Lederhandler and L. G. Giacoletto, Proceedings of the Institute of Radio Engineering, vol. 43 1955, p.477. The fourth is microwave absorption or reflection, which is disclosed in, for example, H. Jacobs, A. P. Ramsa and F. A. Brand, Proceedings of the Institute of Radio Enginerrs, vol. 48, 1960, p.299.
However, there were many problems in the conventional carrier lifetime measuring methods for IV group semiconductors as follows:
First, it was impossible to measure carrier life-time in a minute region, for example, a region having a diameter of about 100 .mu.m or less. Therefore, it was very difficult to know and to control the carrier life-time distribution towards vertical direction of a device (thickness direction). Secondly, it was hardly possible to measure the carrier lifetime with a wide time range of 10 ns to 1 ms. A carrier lifetime changes very much, depending on each stage of the device fabrication process and on the horizontal or vertical positions of a wafer. For example, the carrier lifetime of a raw wafer often takes a high value of about 1 ms. Whereas, the carrier lifetime of the thyristor emitter region sometimes takes a low value of 10 ns because the donor or acceptor concentrations are of the order of 10.sup.19 cm.sup.-3. Therefore, carrier lifetime values to be able to be measured should be very wide. Thirdly, the conventional measuring methods using the electromagnetic wave, such as photo-conductivity and microwave absorption or reflection, can only be used to a wafer. Whereas, the conventional electrical measuring methods, such as reverse recovery time and open circuit voltage decay, can only be used to a diode. There are no methods which can measure the carrier lifetime of all samples from a wafer to a device. This makes it difficult to measure the carrier lifetime change during the device fabrication processes. Fourthly, carrier injection conditions are apt to be limited to a high level injection condition or a low level injection condition in the conventional measuring methods. The carrier lifetime in a high level injection condition coincides with the sum of the minority carrier lifetime and the majority carrier lifetime. Whereas, the carrier lifetime in a low level injection condition is equal to the minority carrier lifetime, which is disclosed in C. T. Sah, R. N. Noice and W. Shockley, Proc. IRE 45, 1957, p.1228. Therefore, the majority carrier lifetime can be obtained by subtracting the low level injection carrier lifetime from the high level injection carrier lifetime. However, this cannot be performed in the conventional carrier lifetime measuring methods because both injection conditions are not attained in the methods.
In the case of a compound semiconductor, such as GaAs and InP, there is another carrier lifetime measuring method besides the above-mentioned ones. A pulse light is irradiated on a compound semiconductor to excite it and to generate excess carriers in it. After the pulse light is turned off, the compound. semiconductor goes from the excited state to a thermal equilibrium state as the excess carriers decrease. A part of excess carriers disappear by emitting a band emission which originates from recombinations of electrons and holes. A carrier lifetime can be measured from the decay time of the band emission, which is disclosed in, for example, J. Christen, D. Bimberg, A. Steckenborn and G. Weimann, Applied Phys. Letters, vol. 44, 1984, p.84.
However, it was very difficult to use the above-mentioned measuring method in order to measure the carrier lifetime of IV group semiconductors from the following reasons. Since IV group semiconductors have indirect bandgaps, the band emission intensity is considerably weak. Furthermore, the peak wavelength of the band emission exists in a region where the sensitivity of a photo-detector, particularly a photo-multiplier, is very low. Therefore, it is required that an extremely strong pulse light, whose photon energy is larger than the bandgaps of IV group semiconductors, is irradiated on IV group semiconductors to measure carrier lifetime correctly. Moreover, it is required that the pulse interval is sufficiently longer than the carrier life-time of IV group semiconductors. Since the conventional light sources, which were used in the carrier lifetime measurement for compound semiconductors, did not satisfy the above-mentioned conditions, this type of measuring method was not able to measure the carrier lifetime of IV group semiconductors.